In recent years, prosthetic devices have been developed and are now in widespread use as attachments, reinforcements or replacements of various members and joints of the human skeletal system. Many modern joint prostheses, including those for replacement of the hip, finger, wrist, elbow and various other joints, are shaped for insertion into the medullary canal of the adjacent bone to secure the prosthesis in place. Synthetic bone cement is one widely used method of securing such prostheses in position within the medullary canal, but it has recently been discovered that problems with this technique include incomplete filling of the cavity of the bone, toxicity of the cement and possible necrosis of the adjacent layer of cancellous or cortical bone. Although bone cement is relatively easy to use in surgical procedures, the above-named problems have prompted development of alternate means of securing prostheses within the medullary canal of the bone adjacent the diseased or damaged joint.
As is well known, bone ingrowth may be accepted by protheses formed of materials having a surface porosity of at least 45 microns. Clinical studies have confirmed that bone ingrowth not only avoids the problems of bone cement, but actually provides better stability of the prosthesis over an extended period of time and may result in improved stress distribution between the prosthesis and adjacent bone. Various prior art patents including U.S. Pat. No. 3,707,006 to Bokros et al and U.S. Pat. No. 3,938,198 to Kahn et al, have recognized the advantage of securing the prosthesis within the medullary canal through bone ingrowth rather than bone cement. As discussed in Bokros et al and Kahn et al, several materials have been utilized to promote bone ingrowth, including porous ceramic coated with pyrolitic carbon, surface treated titanium, cobalt-chrome and stainless steel alloys and various fibrous overlayers such as polysulfone, Teflon.RTM. coated graphite fiber and polyethylene terephthalate woven mesh.
A major limitation of existing prostheses utilizing bone ingrowth rather than bone cement, is that no fastening or securing means are provided to anchor the stem portion of the prosthesis along its entire length within the medullary canal. The Kahn et al patent, for example, discloses a hip joint prosthesis in which the flange of the femoral head prostehsis is pinned to the cortical bone of the femur, but the stem portion is free to move within the medullary canal of the femur. Recent clinical studies have indicated that movement or shifting of the prosthesis within the medullary canal during the initial four to six weeks of postoperative rehabilitation can interrupt or prohibit bone ingrowth thus delaying or severly hampering patient recovery. It can be appreciated that the alternative of keeping a patient essentially immobilized for that period of time to avoid such movement of the prosthesis is very difficult.